Differential column readout scheme for CMOS APS pixels

ABSTRACT

The present invention provides an improved column readout circuitry and method of operation which minimizes substrate and other common mode noise during a read out operation. The circuit improves the consistency of the pixel to pixel output of the pixel array and increases the dynamic range of the pixel output. This is accomplished by obtaining a differential readout of the reset signal and integrated charge signal from a desired pixel along with the reset signal and charge signal from a reference circuit. In this manner common mode noise can be minimized by a combination of signals from the desired and reference pixels in the sample and hold aspect of the column circuitry. In one exemplary embodiment of the invention, a 3T pixel arrangement is used. In another exemplary embodiment, a 4T arrangement is used. Additional exemplary embodiments provide differential column readout circuitry that can be used with any two signal sources.

This application is a divisional of application Ser. No. 11/637,239,filed Dec. 12, 2006, now U.S. Pat. No. 7,525,080, which is acontinuation of application Ser. No. 11/167,076, filed on Jun. 27, 2005,now U.S. Pat. No. 7,154,078, which is a continuation of application Ser.No. 10/230,176, filed Aug. 29, 2002, now U.S. Pat. No. 6,919,551, thedisclosures of which are incorporated by reference in their entireties.

FIELD OF THE INVENTION

The invention relates generally to improved semiconductor imagingdevices and in particular to an imaging device which can be fabricatedusing a standard CMOS process. Particularly, the invention relates to aCMOS active pixel sensor (APS) imager having an array of pixel cells andto the column circuitry for reading the cells.

BACKGROUND OF THE INVENTION

There is a current interest in CMOS active pixel imagers for possibleuse as low cost imaging devices. An exemplary pixel circuit of a CMOSactive pixel sensor (APS) is described below with reference to FIG. 1.Active pixel sensors can have one or more active transistors within thepixel unit cell, can be made compatible with CMOS technologies, andpromise higher readout rates compared to passive pixel sensors. The FIG.1 circuit 100 exemplary pixel cell 150 is a 3T APS, where the 3T iscommonly used in the art to designate use of three transistors tooperate the pixel. A 3T pixel has a photodiode 162, a reset transistor184, a source follower transistor 186, and a row select transistor 188.It should be understood that FIG. 1 shows the circuitry for operation ofa single pixel, and that in practical use there will be an M times Narray of identical pixels arranged in rows and columns with the pixelsof the array accessed using row and column select circuitry, asdescribed in more detail below.

The photodiode 162 converts incident photons to electrons which collectat node A. A source follower transistor 186 has its gate connected tonode A and thus amplifies the signal appearing at Node A. When aparticular row containing cell 150 is selected by a row selectiontransistor 188, the signal amplified by transistor 186 is passed on acolumn line 170 to the readout circuitry. The photodiode 162 accumulatesa photo-generated charge in a doped region of the substrate. It shouldbe understood that the CMOS imager might include a photogate or otherphotoconversion device, in lieu of a photodiode, for producingphoto-generated charge.

A reset voltage source Vrst is selectively coupled through resettransistor 184 to node A. The gate of reset transistor 184 is coupled toa reset control line 190 which serves to control the reset operation inwhich Vrst is connected to node A. The row select control line 160 iscoupled to all of the pixels of the same row of the array. Voltagesource Vdd is coupled to a source following transistor 186 and itsoutput is selectively coupled to a column line 170 through row selecttransistor 188. Although not shown in FIG. 1, column line 170 is coupledto all of the pixels of the same column of the array and typically has acurrent sink at its lower end. The gate of row select transistor 188 iscoupled to row select control line 160.

As know in the art, a value is read from pixel 150 in a two stepprocess. During a charge integration period the photodiode 162 convertsphotons to electrons which collect at the node A. The charges at node Aare amplified by source follower transistor 186 and selectively passedto column line 170 by row access transistor 188. During a reset period,node A is reset by turning on reset transistor 184 and the reset voltageis applied to node A and read out to column line 170 by the sourcefollower transistor 186 through the activated row select transistor 188.As a result, the two different values—the reset voltage and the imagesignal voltage—are readout from the pixel and sent by the column line170 to the readout circuitry where each is sampled and held for furtherprocessing as known in the art.

All pixels in a row are read out simultaneously onto respective columnlines 170 and stored in sample and hold circuits. Then the columncircuitry in the sample and hold circuits are activated in sequence forreset and signal voltage read out. The rows of pixels are also read outin sequence onto the respective column lines.

FIG. 2 shows a CMOS active pixel sensor integrated circuit chip thatincludes an array of pixels 230 and a controller 232 which providestiming and control signals to enable reading out of signals stored inthe pixels in a manner commonly known to those skilled in the art.Exemplary arrays have dimensions of M times N pixels, with the size ofthe array 230 depending on a particular application. The imager is readout a row at a time using a column parallel readout architecture. Thecontroller 232 selects a particular row of pixels in the array 230 bycontrolling the operation of row addressing circuit 234 and row drivers240. Charge signals stored in the selected row of pixels are provided onthe column lines 170 (FIG. 1) to a readout circuit 242 in the mannerdescribed above. The pixel signal read from each of the columns then canbe read out sequentially using a column addressing circuit 244.Differential pixel signals (Vrst, Vsig) corresponding to the read outreset signal and integrated charge signal are provided as respectiveoutputs Vout1, Vout2 of the readout circuit 242.

FIG. 3 more clearly shows the rows and columns 349 of pixels 350. Eachcolumn includes multiple rows of pixels 350. Signals from the pixels 350in a particular column can be read out to a readout circuit 352associated with that column. The read out circuit 352 includes sampleand hold circuitry for acquiring the pixel reset and integrated chargesignals. Signals stored in the readout circuits 352 then can be readsequentially column-by-column to an output stage 354 which is common tothe entire array of pixels 330. The analog output signals can then besent, for example, to a differential analog circuit and which subtractsthe reset and integrated charge signals and sends them to ananalog-to-digital converter (ADC), or the reset and integrated chargesignals are each supplied to the analog-to-digital converter.

FIG. 4 more clearly shows the column readout circuit 352 includes thesample and hold read out circuit in the prior art. The sample and holdcircuit 400 is capable of sampling and holding two signals forsubsequent subtraction. For example, a reset signal would be stored oncapacitor 418 and the charge accumulated photo signal would be stored oncapacitor 420. A downstream circuit subtracts these signals and outputsa signal to a digital to analog converter.

The capacitors 418, 420 are typically clamped to a clamping voltage ontheir back sides by switch 415 before a sampling operation. To store thepixel image signal on capacitor 420, a pulse signal is applied whichtemporarily closes the SHS switch 412 and couples the pixel with thefront side of capacitor 420 through the column line 402. Thereafter, theSHS switch 412 is opened, which retains the charge accumulated signal inthe capacitor 420 (assuming that Col. Select switch 428 is open).Similarly, to store the reset signal on capacitor 418, a pulse signaltemporarily closes the SHR switch 410 and couples the pixel with thefront side of capacitor 418 through the column line 402. Thereafter, theSHR switch 410 is opened, which retains the reset signal in thecapacitor 418 (assuming that Col. Select switch 426 is open).

In order to read out the stored reset and charge accumulated signalsfrom the capacitors 418, and 420 a pulse signal is applied closing acrowbar switch 413 and Col. Select switches 426 and 428 thereby forcingsignals on capacitors 418, 420 into differential amplifier 434. Signalsoutput from amplifier 434 are provided to downstream circuits. Althoughamplifier 434 is shown as processing the reset and image signals toprovide differential signals to downstream circuits, amplifier 434 canalso be arranged to subtract the signals and provide single endedsignals to downstream processing circuits.

As fabrication techniques get better, an increasing number of digitalprocessing circuits are being implemented on the same chip as an imagesensor. This increases substrate noise coupling to a pixel, which cancompromise the signal to noise ratio of the image sensor core. Thesubstrate noise occurs when spurious noise signals are injected locallyinto the substrate through ohmic or capacitive coupling, therebybreaking the equipotentiality of the substrate.

In traditional CMOS APS devices the two signals, corresponding to theimage signal level (Vsig) and the reset signal level (Vrst) are read outof each pixel at two different times. The Vsig and Vrst voltages storedon the respective capacitors 420, 418 with reference to the prechargeclamping voltage Vclamp. Thus,Vsig=Vsig−Vclamp  (1)

where Vclamp is the clamping voltage. Likewise,Vrst=Vrst−Vclamp  (2)

During readout the difference between Vsig and Vrst voltages isgenerated as an output. Ideally, this will be:Vdiff=Vrst−Vsig  (3)

However, in practice there will be an uncorrelated noise componentassociated with Vclamp, giving an actual output voltage ofVdiff=Vrst−Vsig−(Vclamp(rst)−Vclamp(sig))  (4)

It would be desirable to have a column readout circuit that compensatesfor substrate and other common mode noise that is encountered during apixel read out operation and eliminates the noise term.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an improved column readout circuitry andmethod of operation which minimizes substrate and other common modenoise during a read out operation. The circuit improves the consistencyof the pixel to pixel output of the pixel array and increases thedynamic range of the pixel output. This is accomplished by obtaining adifferential readout of the reset signal and image signal from a desiredpixel along with the reset signal and charge signal from a referencecircuit. In this manner common mode noise seen by both can be minimizedby a combination of signals from the desired pixel and reference circuitin the sample and hold portion of the column circuitry. In one exemplaryembodiment of the invention, a 3T pixel arrangement is used. In anotherexemplary embodiment, a 4T arrangement is used. Additional exemplaryembodiments provide differential column readout circuitry that can beused with any two signal sources.

These and other features and advantages of the invention will be morereadily understood from the following detailed description of theinvention which is provided in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a prior art active pixel;

FIG. 2 is a block diagram of a prior art CMOS active sensor chip;

FIG. 3 is a block diagram of a prior art array of active pixels and anassociated readout circuit;

FIG. 4 is a prior art sample and hold circuit;

FIG. 5 is a differential readout pixel circuit and associated columnreadout circuitry in accordance with an exemplary embodiment of theinvention;

FIG. 6 is a simplified timing diagram associated with the circuitry ofFIG. 5;

FIG. 7 is a differential readout pixel circuit and associated columnreadout circuitry in accordance with an another exemplary embodiment ofthe invention;

FIG. 8 is a simplified timing diagram associated with the circuitry ofFIG. 7;

FIG. 9 is a differential column readout circuit in accordance withanother exemplary embodiment of the invention;

FIG. 10 is a simplified timing diagram associated with the circuitry ofFIG. 9;

FIG. 11 is a differential column readout circuit in accordance withanother exemplary embodiment of the invention;

FIG. 12 is a simplified timing diagram associated with the circuitry ofFIG. 11;

FIG. 13 is a differential column readout circuit in accordance withanother exemplary embodiment of the invention;

FIG. 14 is a simplified timing diagram associated with the circuitry ofFIG. 13;

FIG. 15 is a differential column readout circuit in accordance withanother exemplary embodiment of the invention;

FIG. 16 is a simplified timing diagram associated with the circuitry ofFIGS. 15 and 19;

FIG. 17 is a differential column readout circuit in accordance withanother exemplary embodiment of the invention;

FIG. 18 is a simplified timing diagram associated with the circuitry ofFIG. 17;

FIG. 19 is a differential column readout circuit in accordance withanother exemplary embodiment of the invention;

FIG. 20 is a differential column readout circuit in accordance withanother exemplary embodiment of the invention;

FIG. 21 is a simplified timing diagram associated with the circuitry ofFIG. 20;

FIG. 22 is a differential column readout circuit in accordance withanother exemplary embodiment of the invention;

FIG. 23 is a simplified timing diagram associated with the circuitry ofFIG. 22; and

FIG. 24 is a block diagram representation of a processor-based systemincorporating a CMOS imaging device in accordance with an exemplaryembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. These embodiments are described in sufficient detail toenable those of ordinary skill in the art to make and use the invention,and it is to be understood that structural, logical or other changes maybe made to the specific embodiments disclosed without departing from thespirit and scope of the present invention.

To minimize additional new noise, the present invention utilizes adifferential column readout circuit using two signal sources—a desiredpixel and a reference circuit—as the sources of reference differentialsignals which are combined and output from the column readout circuit504. In an exemplary embodiment, shown in FIG. 5, there is no actualphotosignal from the reference circuit 540 available; however acomparison value from the reference circuit 540 is taken, which is takenat the same time of taking the photo signal of the desired pixel 550.The reset signal of the reference circuit 540 is taken at the same timethat the reset signal of the desired pixel 550 is taken. Since both thedesired pixel 550 and the reference circuit 540 see the same biasvoltage, any induced ground noise or other common mode noise is alsosubstantially the same. By subtracting sampled signal values of thereference circuit 540 from sampled signal values from the desired pixel550 and factoring in biases, the common noise is cancelled out andremoved.

Column readout circuit 504 provides three bias sources ensuring adifferential output. The bias sources are a pbias current source 575, atail bias current source 578, and a common mode feedback (CMF) currentbias source 580. The pbias component, introduced through two pbiascurrent transistors 574, 576, is equivalent to the tail current biaswhich is introduced through tail current bias transistor 578. A CMFcurrent bias, introduced through CMF current bias transistor 580,compensates for any difference between the tail current bias transistor578 and the two pbias current transistors 574, 576. The combination ofthese three biases permit differential sampling of the desired pixel 550and the reference circuit 540.

FIG. 5 illustrates a fully differential pixel readout architecture 500in accordance with the present invention. This architecture has twocomponents—the circuitry in the pixel 550 and the circuitry in thecolumn readout 504. This combination of these components permits a fullydifferentiated column sampling and readout. As noted, the column readoutcircuit 504 includes a reference circuit 540 acting as a “dummy” pixel,a sample and hold circuit 538, an amplifier 534 and three bias sources(e.g., pbias current circuit 575, tail current bias transistor 578, andCMF current bias transistor 580).

The pixel 550 is different from the prior art 3T pixel described inconnection with FIG. 1 in that a capacitor 551 is added and thearchitecture is rearranged. The exemplary pixel cell 550 forms a 3T APS,which has a photodiode 562, a reset transistor 584, a source followertransistor 586, a row select transistor 588 and the capacitor 551.Photodiode 562 is coupled to Node A and Node A is coupled to the gate ofsource follower transistor 586. A source/drain region of a sourcefollower transistor 586 is coupled through conductive line 572 to tailcurrent bias transistor 578 and a CMF current bias transistor 580. Theother source/drain region of source follower transistor 586 is coupled asource/drain of row selection transistor 588. The other source/drainregion of row selection transistor 588 is coupled to a column line 570.Node A is also coupled to a source/drain region of a reset transistor584 and the other source/drain of reset transistor 584 is coupledbetween row selection transistor 588 and the source follower transistor586. Capacitor 551 is coupled on one side to Node A and on the otherside to the interconnection of row selection transistor 588 and sourcefollower transistor 586.

Reference circuit 540 within the column readout circuit 504 is a similarcircuit to the pixel 550 but lacks a photodiode 562. The referencecircuit 540 consists of a (second) Node A′ coupled to the gate of sourcefollower transistor 546. A source/drain region of a source followertransistor 546 is coupled to tail bias current transistor 578 and CMFcurrent bias transistor 580 and the other source/drain region is coupleda source/drain of reference circuit selection transistor 548. The othersource/drain region of reference circuit selection transistor 548 iscoupled to pbais circuit 575. Node A′ is also coupled to a source/drainregion of a reset transistor 544 and the other source/drain of resettransistor 544 is coupled between row selection transistor 548 and thesource follower transistor 546. Capacitor 541 is coupled on one side toNode A′ and on the other side to the connection between source followertransistor 546 and reset transistor 544.

Coupled in between pixel 550 and the reference circuit 540 is thepbiasing circuit 575, which consists of Vdd coupled in between a pair ofpbias transistors 574, 576.

In this embodiment, the sample and hold circuit 538 is capable ofsampling and holding two sets of signals simultaneously, e.g., two resetsignals and two photo signals, one photo and one comparison signal,respectively, from a desired pixel and the reference circuit, and thensubsequently combining the two sets of signals. For example, a resetsignal of the desired pixel 550 is sampled and stored on capacitor 514and at the same time a reset signal of the reference circuit 540 isstored on capacitor 518. Similarly, the charge accumulated photo signalof the desired pixel 550 is sampled and stored on capacitor 516 and atthe same time a comparison signal for the reference circuit 540 issampled and stored on capacitor 520.

The sample and hold circuit 538 has a first signal input line 554 whichis switchably coupled through respective SHR switch 506 and SHS switch508 to a first side of respective capacitors 514, 516. The other side ofcapacitors 514, 516 are switchably coupled through respective switch526, 528 to a first and second input to amplifier 534. The sample andhold circuit 538 has a second signal input line 556 which is switchablycoupled through respective SHR switch 510 and SHS switch 512 to a firstside of respective capacitors 518, 520. The other side of respectivecapacitors 518, 520 is respectively coupled between the other side ofcapacitors 514, 516 and respective switches 526, 528.

A first crowbar switchably couples through a switch 522 the conductiveline between the SHR switch 506 and capacitor 514 to the conductive linebetween the SHR switch 510 and capacitor 518. A second crowbarswitchably couples through a switch 524 the conductive line between theSHS switch 508 and capacitor 516 to the conductive line between the SHSswitch 512 and capacitor 520. A Vclamp voltage is switchably coupledthrough a switch 532 to the conductive line between the capacitor 514and the switch 526. A Vclamp voltage is also switchably coupled througha switch 530 to the conductive line between the capacitor 516 and theswitch 528. The first signal input line 554 to the sample and holdcircuit 538 is coupled to the connection between the pbiasing circuit575 and the pixel array 502. The second signal input line 556 to thesample and hold circuit 538 is coupled to the column line 570 in thereference circuit 540 between the reference selection transistor 548 andthe source follower transistor 546.

The operation of the FIG. 5 circuits is now described with reference tothe simplified signal timing diagram of FIG. 6. Switches 530, 532 arefirst pulsed, by pulse Clamp, thereby temporarily coupling Vclamp to thebackside of capacitors 514, 516, 518, and 520, and placing an initialcharge on them. The row selection transistor 588, the reference circuitselection transistor 548, and SHS switches 508, 512 are then pulsed onby pulses Sel, Ref Sel, and SHS. The desired pixel 550 and referencecircuit 540 signal voltages are sampled by a sample and hold circuit 538connected to the column line 570. SHS switches 508, 512 are thendisabled. Thus, Vsig of the pixel 550 is sampled and stored on capacitor516, and the Vsig of the reference circuit 540 is sampled and stored oncapacitor 520. The reset transistors 584, 544 are temporarily enabled bypulses Reset, Ref Reset. A pulse SHR temporarily enables SHR switches506, 510. Pulses Sel, Ref Sel are also applied enabling the rowselection transistor 588 and the reference circuit selection transistor548. Thus, Vrst of the desired pixel 550 is sampled and stored oncapacitor 514 and Vrst of the reference circuit 540 is sampled andstored on capacitor 518. Capacitor 516 connected to the pixel 550 storesthe signal voltage of the pixel 550 at the same time capacitor 520connected to the reference circuit 540 stores the comparison signal ofthe reference circuit 540. Additionally, capacitor 514 connected to thepixel 550 stores the reset signal voltage of the pixel 550 at the sametime capacitor 518 connected to the reference circuit 540 stores thereset signal of the reference circuit 540.

In order to read out the stored signals of the pixel 550 and referencecircuit 540 from the capacitors 514, 516, 518, and 520, pulses Col Sel,CB signal are applied temporarily closing Col. Select switches 526, 528and crowbar switches 522, 524 and thereby coupling capacitors 514, 518with a first signal input line to differential amplifier 534 andcoupling capacitors 516, 520 with a second signal input line todifferential amplifier 534. Amplifier 534 outputs the resultingdifferential signals.

In this manner four sampled signals, Vsig(desired pixel), Vsig(referencecircuit), Vrst(desired pixel) and Vrst(reference circuit), are sampledand stored on respective capacitors 514, 516, 518, and 520. By samplingVsig(reference circuit) at the same time as sampling Vsig(desiredpixel), Vsig(reference circuit) is used to subtract out the noise inVsig(desired pixel). Similarly, by sampling Vrst(reference circuit) atthe same time as sampling Vrst(desired pixel), Vrst(reference circuit)is used to subtract out the noise in Vrst(desired pixel). These storedsignals are then combined to provide:Vdiff=(Vsig(pixel)−Vsig(reference))−(Vrst(pixel)−Vrst(reference)).  (5)

where this fully differentiated charge output Vdiff of amplifier 534 isfree of common noise.

It should be understood that while FIG. 5 shows the circuitry foroperation of a single pixel, that in practical use there will be an Mtimes N array of pixels arranged in rows and columns.

FIG. 7 illustrates another exemplary embodiment of a fully differentialpixel readout architecture 700 of the present invention. The FIG. 7differential pixel readout architecture 700 differs from thedifferential pixel readout architecture 500 of FIG. 5 in that a modified4T pixel 750 is used in place of the 3T pixel 550 of FIG. 5. Therefore,the only difference in the circuitry between the two architectures 500,700 is a transfer control transistor 796 between the photodiode 762 andNode A of the desired pixel 750 which is operated at the end of thecharge integration period to transfer the accumulated charge atphotodiode 762 to node A.

The operation of pixel readout architecture 700 is similar to theoperation of pixel readout architecture 500 except that transfertransistor 796 gets activated to couple the photodiode 762 to Node A. Anadditional difference is that order of reading charge integrated andresets signals from a desired 4T pixel 750 is reversed from the order ofreading from a desired 3T pixel 550. FIG. 8 provides simplified signaltiming diagram for the method of operating pixel readout architecture700.

FIG. 9 illustrates another exemplary embodiment of a differential columnstorage and readout circuit 900 of the present invention. FIG. 9 is amore generalized sample and hold circuit which can be used with adesired pixel and a reference circuit from elsewhere to eliminate noisein the desired pixel signal, where the desired pixel signals Vsig andVrst enter on input line Vin1 and the reference circuit signals Vsig andVrst enter on input line Vin2.

Sample and hold circuit 938 stores each pair of signals on a capacitor.For example, the Vsig of the desired pixel and the Vsig of the referencecircuit are stored on C1 capacitor 914 and Vrst of the desired pixel andVrst of the reference circuit are stored on C2 capacitor 916. In thisway the charge on the first capacitor isC1=(Vsig1−Vsig2)  (6)

and the charge on the second capacitor isC2=(Vrst1−Vrst2).  (7)

Consequently, the readout charge injected into the amplifier 934 is:Vdiff=((Vsig1−Vsig2)−(Vrst1−Vrst2))  (8)

The differential output of the amplifier provides elimination of a noisecomponent in the difference signal Vdiff derived from substrate andground noise which is common to the desired pixel and reference circuit.

The sample and hold circuit 938 has a first signal input line 954 from afirst signal source which is switchably coupled through respective shr1reset sample switch 906 and shs1 signal sample switch 908 to a firstside of respective C2 capacitor 916, C1 capacitor 914. The sample andhold circuit 938 has a second signal input line 956 from a second signalsource which is switchably coupled through respective shr2 reset sampleswitch 910 and shs2 signal sample switch 912 to a second side ofrespective capacitors 916, 914. The second side of capacitor 914 isswitchably coupled to a first input to amplifier 934 through columnselect switch 926. The second side of capacitor 916 is switchablycoupled to a second input to amplifier 934 through column select switch928.

Although amplifier 934 is shown in FIG. 9 in proximity to the columnsample and hold circuit 938, amplifier 934 may be globally accessibleand therefore implemented outside of the column readout circuit 900. Inanother aspect of the invention, column readout circuit 900 includesamplifier 934.

A crowbar switchably couples through a switch 922 the conductive linebetween the shr1 reset sample switch 906 and capacitor 916 to theconductive line between the shs1 signal sample switch 908 and capacitor914.

The operation of circuit 938 is now described with reference to thesimplified signal timing diagram of FIG. 10 (implementing a read outfrom a 3T pixel).

The shs1 signal sample switch 908 and shs2 signal sample switch 912 aretemporarily enabled by pulses shs1, shs2 and are then disabled. Thus,Vsig of a desired pixel and Vsig of a reference circuit are sampled andstored on capacitor 914. Pulses shr1, shr2 are applied temporarilyenabling the shr1 reset sample switch 906 and shr2 reset sample switch910. Thus, Vrst of a desired pixel and Vrst of a reference circuit aresampled and stored on capacitor 916. Thus, capacitor 914 stores thesignal voltage of the desired pixel at the same time it stores thesignal of the reference circuit. Additionally, capacitor 916 stores thereset signal voltage of a desired pixel at the same time it stores thereset signal of a reference circuit.

In order to read out the stored signals of a desired pixel and referencecircuit from the capacitors 914, 916, a pulse signal is appliedtemporarily closing Col. Select switches 926, 928. Crowbar switch 922 ispulsed thereby forcing signals on capacitors 914, 916 throughdifferential amplifier 934. Amplifier 934 outputs the resultingdifferential signals, thereby providing a fully differential output inwhich common source noise is eliminated from the desired pixel outputsignal.

FIG. 11 illustrates another exemplary embodiment of a differentialcolumn storage and readout circuit 1100 of the present invention. TheFIG. 11 is similar to the FIG. 9 circuit, but circuit 1100 does not usethe crowbar circuit 922 (FIG. 9).

The sample and hold circuit 1138 has a first signal input line 1154 froma first signal source which carries the Vsig and Vrst signals of thedesired pixel. The first input line 1154 is switchably coupled throughrespective shr1 reset sample switch 1106 and shs1 signal sample switch1108 to a first side of respective capacitors 1116, 1114. The sample andhold circuit 1138 has a second signal input line 1156 from a secondsignal source which carries the Vsig and Vrst signals of the referencecircuit. The second signal input line 1156 is switchably coupled throughrespective shr2 reset sample switch 1110 and shs2 signal sample switch1112 to a second side of respective capacitors 1116, 1114.

A Vclamp voltage is switchably coupled through a shs2 signal sampleswitch 1130 to the conductive line between the capacitor 1114 and theswitch 1112. A Vclamp voltage is also switchably coupled through a shr2reset sample switch 1132 to the conductive line between the capacitor1116 and the switch 1110. The clamp voltage is used to precharge thecapacitors 1116, 1114 before sampling.

The first side of capacitor 1114 is switchably coupled to a first sideof a capacitor 1190 through column select switch 1126. The back side ofcapacitor 1190 is coupled to the first input of amplifier 1134. A Vclampvoltage is switchably coupled through a switch 1180 to the first side ofcapacitor 1190. The conductive line between the switch 1126 andcapacitor 1190 is coupled through a capacitor 1196 to the first outputof the amplifier 11134. The conductive line between capacitor 1190 andamplifier 1134 is switchably coupled through switch 1186 to theconductive line between capacitor 1196 and the first output of amplifier1134.

The first side of capacitor 1116 is switchably coupled to a first sideof a capacitor 1192 through column select switch 1128. The back side ofcapacitor 1192 is coupled to the second input of amplifier 1134. AVclamp voltage is switchably coupled through a switch 1182 to the firstside of capacitor 1192. The conductive line between the switch 1128 andcapacitor 1192 is coupled through a capacitor 1194 to the second outputof the amplifier 1134. The conductive line between capacitor 1192 andamplifier 1134 is switchably coupled through switch 1188 to theconductive line between capacitor 1194 and the second output ofamplifier 1134.

The operation of circuit 1138 is now described with reference to thesimplified signal timing diagram of FIG. 12 (implementing a read outfrom a 3T pixel).

The shs1 signal sample switch 1108 and shs2 signal sample switch 1112are pulsed by pulses shs1, shs2. Thus, Vsig of a pixel and Vsig of areference circuit are sampled and stored on capacitor 1114. Pulses shr1,shr2 are applied enabling shr1 reset sample switch 1106 and shr2 resetsample switch 1110. Thus, Vrst of a pixel and Vrst of a referencecircuit are sampled and stored on capacitor 1116. Capacitor 1114 storesthe signal voltage of the pixel at the same time it stores the signal ofthe reference circuit. Additionally, capacitor 1116 stores the resetsignal voltage of the pixel at the same time it stores the reset signalof a reference circuit.

In order to read out the stored signals, the amplifier 1134 is reset.Therefore pulses Cf1, Cf2, f1, f2 are sent temporarily enabling switches1180, 1182, 1186, and 1188. To read the stored signals of a pixel andreference circuit from the capacitors 1114, 1116, a pulse Clamp isapplied to switches 1130, 1132 temporarily coupling Vclamp to thecapacitors 1114, 1116. A pulse Col_Sel signal is sent temporarilyclosing Col. Select switches 1126, 1128 thereby forcing the signals oncapacitors 1114, 1116 through differential amplifier 1134. Amplifier1134 outputs the resulting differential signals, thereby providing afully differential output in which common source noise is eliminatedfrom the desired output signal.

FIG. 13 illustrates another exemplary embodiment of a differentialcolumn storage and readout circuit 1300 of the present invention.

The architecture of circuit 1300 is similar to that of circuit 900 inFIG. 9, in that circuit 1300 reads in two pairs of voltages and storesthem on four respective capacitors, 1314, 1316, 1318, and 1320. However,circuit 1300 combines signals that are mixed to eliminate noise betweentwo pairs of signals prior to them being input to a differentialamplifier. For example, Vsig of the desired pixel is combined with theVrst of the reference circuit, providing:Vout1=½(Vsig(pixel)+Vrst(reference)).  (9)

Vrst of the desired pixel is combined with the Vsig of the referencecircuit, providingVout2=½(Vsig(reference)+Vrst(pixel)).  (10)

Therefore, the voltage difference can be determined using knowntechniques to be:Vdiff=½((Vsig(pixel)−Vrst(pixel))−(Vsig(reference)−Vrst(reference)).  (11)

Sample and hold circuit 1338 stores each signal of the two pairs ofsignals on a respective capacitor 1314, 1316, 1318, and 1320. Forexample, the Vsig of the desired pixel and the Vsig of the referencecircuit are stored on a respective capacitor 1314, 1318 and the Vrst ofthe desired pixel and the Vrst of the reference circuit are stored on arespective capacitor 1316, 1320. The differential output of theamplifier provides elimination of a common noise component derived fromsubstrate and ground noise.

The sample and hold circuit 1338 has a first signal input line 1354 froma first signal source which is switchably coupled through respectiveshr1 reset sample switch 1306 and shs1 signal sample switch 1308 to afirst side of respective capacitors 1316, 1314. The second side ofcapacitors 1316, 1314 are coupled to ground. The sample and hold circuit1338 has a second signal input line 1356 from a second signal sourcewhich is switchably coupled through respective shr2 reset sample switch1310 and shs2 signal sample switch 1312 to a second side of respectivecapacitors 1320, 1318. The other side of respective capacitors 1320,1318 are coupled to ground.

The first side of capacitors 1314, 1320 are switchably coupled to afirst signal input line to a downstream amplifier through respectiveswitches 1326, 1327. The first side of capacitors 1316, 1318 areswitchably coupled to a second signal input line to a downstreamamplifier through respective switches 1328, 1329.

The operation of circuit 1338 is now described with reference to thesimplified signal timing diagram of FIG. 14 (implementing a read from a3T pixel).

The shs1 signal sample switch 1308 and shs2 signal sample switch 1312are temporarily enabled with pulses shs1, shs2 signals. Thus, Vsig of adesired pixel and Vsig of a reference circuit are sampled and stored onrespective capacitors 1314, 1318. Pulses shr1, shr2 are appliedtemporarily enabling shr1 reset sample switch 1306 and shr2 reset sampleswitch 1310. Thus, Vrst of a desired pixel and Vrst of a referencecircuit are sampled and stored on respective capacitors 1316, 1320.Capacitor 1314 stores the signal voltage of a desired pixel at the sametime it stores the signal of the reference circuit on capacitor 1318.Additionally, capacitor 1316 stores the reset signal voltage of adesired pixel at the same time the reset signal of a reference circuitis stored on capacitor 1320.

To read out the stored signals from the capacitors 1314, 1320 an enablepulse signal is applied temporarily closing switches 1326, 1327 therebyforcing the signals stored on capacitors 1314, 1320 through a firstinput of a downstream amplifier. The enable pulse signal is also appliedto and closes switches 1328, 1329 thereby forcing the signals stored oncapacitors 1316, 1318 through a second input of a downstream amplifier.The amplifier outputs the resulting differential signals, therebyproviding a fully differential output in which common source noise iseliminated from the desired output signal.

FIG. 15 illustrates another exemplary embodiment of a differentialcolumn storage and readout circuit 1500 of the present invention. Sampleand hold circuit 1538 reads in two pairs of signals, e.g., Vsig(pixel),Vrst(pixel), Vsig(reference), and Vrst(reference), and stores them onfour respective capacitors 1514, 1516, 1518, and 1520. The sample andhold circuit 1538 combines mixed signals to eliminate common noisebetween two pairs of signals prior to being input into the downstreamamplifier 1534. For example, Vsig of the desired pixel is combined withVrst of the reference circuit and the input into the first input of anamplifier. Vrst of the desired pixel is combined with Vsig of thereference circuit and input into the second input of an amplifier.

The sample and hold circuit 1538 has a first signal input line 1554 froma first signal source which is switchably coupled through respectiveshr1 reset sample switch 1506 and shs1 signal sample switch 1508 to afirst side of respective capacitors 1516, 1514. The sample and holdcircuit 1538 has a second signal input line 1556 from a second signalsource which is switchably coupled through respective shr2 reset sampleswitch 1510 and shs2 signal sample switch 1512 to a first side ofrespective capacitors 1520, 1518. The back side of respective capacitors1520, 1514 are switchably coupled to a first signal input line ofamplifier 1534 through column select switch 1526. The back side ofrespective capacitors 1518, 1516 are switchably coupled to a secondsignal input line of amplifier 1534 through column select switch 1528.Signal line 1554 is coupled to a desired pixel while signal line 1556 iscoupled to a reference circuit.

A crowbar switchably couples through a switch 1522 the conductive linebetween the shr1 reset sample switch 1506 and capacitor 1516 to theconductive line between the shs1 signal sample switch 1508 and capacitor1514. A second crowbar switchably couples through a switch 1523 theconductive line between the shr2 reset sample switch 1510 and capacitor1520 to the conductive line between the shs2 signal sample switch 1512and capacitor 1518. A Vclamp voltage is switchably coupled through aswitch 1530 to the connection between the capacitor 1520 and thecapacitor 1514. A Vclamp voltage is also switchably coupled through aswitch 1532 to the connection between capacitor 1516 and capacitor 1518.The clamp voltage is used to precharge the capacitors 1514, 1516, 1518,and 1520.

The first signal input line to the amplifier 1534, i.e., the conductiveline between the column select switch 1526 and amplifier 1534, iscoupled to the first output from amplifier 1534 through capacitor 1596.The first signal input line to the amplifier 1534 is also switchablycoupled to the first output from amplifier 1534 through switch 1586. Thesecond signal input line to the amplifier 1534, i.e., the conductiveline between the column select switch 1528 and amplifier 1534, iscoupled to the second output from amplifier 1534 through capacitor 1594.The second signal input line to the amplifier 1534 is also switchablycoupled to the second output from amplifier 1534 through switch 1588.

The operation of circuit 1538 is now described with reference to thesimplified signal timing diagram of FIG. 16 (implementing a read outfrom a 3T pixel).

The Vclamp switches 1508, 1506 are enabled by a pulse Clamp, therebyprecharging the capacitors 1514, 1516, 1518, and 1520. The shs1 signalsample switch 1508 and shs2 signal sample switch 1512 are pulsed bypulses shs1, shs2 thereby temporarily enabling them. Thus, Vsig of adesired pixel and Vsig of a reference circuit are sampled and stored onrespective capacitors 1514, 1518. Pulses shr1, shr2 are appliedtemporarily enabling shr1 reset sample switch 1506 and shr2 reset sampleswitch 1510. Thus, Vrst of a desired pixel and Vrst of a referencecircuit are sampled and stored on respective capacitors 1516, 1520.

In order to read out the stored signals from the capacitors 1514, 1516,1518, and 1520, the amplifier 1534 is reset by temporarily closingswitches 1594 and 1596. A pulse ColSel signal is applied temporarilyclosing Col. Select switches 1526, 1528 thereby coupling capacitors1514, 1516, 1518, and 1520 to differential amplifier 1534. Crowbarswitches 1522, 1523 are pulsed by CB closed thereby forcing the storedsignals in capacitors 1514, 1516, 1518, and 1520 through amplifier 1534.Amplifier 1534 outputs the resulting differential signals, therebyproviding a fully differential output in which common source noise iseliminated.

FIG. 17 illustrates another exemplary embodiment of a differentialcolumn storage and readout circuit 1700 of the present invention. Thecircuit 1700 architecture is different from the column storage andreadout circuit 1500 (FIG. 15) architecture in that the circuit 1700does not include a Vclamp circuit, which alters the method of operatingthe circuit.

The sample and hold circuit 1738 has a first signal input line 1754 froma first signal source which is switchably coupled through respectiveshr1 reset sample switch 1706 and shs1 signal sample switch 1708 tofirst sides of respective capacitors 1716, 1714. The sample and holdcircuit 1738 has a second signal input line 1756 from a second signalsource which is switchably coupled through respective shr2 reset sampleswitch 1710 and shs2 signal sample switch 1712 to first sides ofrespective capacitors 1720, 1718. The back sides of respectivecapacitors 1720, 1714 are switchably coupled to a first signal inputline of amplifier 1734 through column select switch 1726. The back sidesof respective capacitors 1718, 1716 are switchably coupled to a secondsignal input line of amplifier 1734 through column select switch 1728.Signal input line 1754 is coupled to a desired pixel, and signal inputline 1756 is coupled to a reference circuit.

A crowbar switch 1722 couples on one side the connection between shr1reset sample switch 1706 and capacitor 1716 and on the other side theconnection between shs1 signal sample switch 1708 and capacitor 1714. Asecond crowbar switch 1723 couples on one side the connection betweenshr2 reset sample switch 1710 and capacitor 1720 and on the other sidethe connection between shs2 signal sample switch 1712 and capacitor1718.

The first signal input line to the amplifier 1734, i.e., the conductiveline between the column select switch 1726 and amplifier 1734, iscoupled to the first output from amplifier 1734 through a capacitor1796. The first signal input line to the amplifier 1734 is alsoswitchably coupled to the first output from amplifier 1734 throughswitch 1786. The second signal input line to the amplifier 1734, i.e.,the conductive line between the column select switch 1728 and amplifier1734, is coupled to the second output from amplifier 1734 through acapacitor 1794. The second signal input line to the amplifier 1734 isalso switchably coupled to the second output from amplifier 1734 throughswitch 1788.

The operation of circuit 1738 is now described with reference to thesimplified signal timing diagram of FIG. 18 (implementing a read outfrom a 3T pixel).

During signal samples, the switches 1788, 1786 and col. sel. switches1728, 1726 are respectively enabled by pulses f1, f2 and col. sel. Theshs1 signal sample switch 1708 and shs2 signal sample switch 1712 arepulsed by shs1, shs2, thereby enabling them. Thus, Vsig of a desiredpixel and Vsig of a reference circuit are sampled and stored onrespective capacitors 1714, 1718. Pulses shr1, 2 are applied enablingshr1 reset sample switch 1706 and shr2 reset sample switch 1710. Thus,Vrst of a desired pixel and Vrst of a reference circuit are sampled andstored on respective capacitors 1716, 1720. Thereafter, the pulses f1,f2, ColSel to switches 1788, 1786, 1728, and 1726 are disabled.

To read out the stored signals of a desired pixel and reference circuitfrom the capacitors 1714, 1716, 1718, and 1720, switches 1788, 1786 andcol. sel. switches 1728, 1726 are closed by pulses f1, f2, ColSel,thereby coupling capacitors 1714, 1720 to a first signal input line ofdifferential amplifier 1734 and coupling capacitors 1716, 1718 to asecond signal input line of differential amplifier 1734. Then a pulse CBis temporarily applied closing crowbar switches 1722, 1723, therebyforcing the signals stored in the capacitors, 1714, 1716, 1718, and 1720through amplifier 1734. Amplifier 1734 outputs the resultingdifferential signals, thereby providing a fully differential output inwhich common mode noise is eliminated from the desired pixel outputsignal.

FIG. 19 illustrates another exemplary embodiment of a differentialcolumn storage and readout circuit 1900 of the present invention. TheFIG. 19 differential column storage and readout circuit 1900 is similarto the circuit 1500 of FIG. 15 but adds an additional crowbar circuit1921.

The sample and hold circuit 1938 has a first signal input line 1954 froma first signal source which is switchably coupled through respectiveshr1 reset sample switch 1906 and shs1 signal sample switch 1908 tofirst sides of respective capacitors 1916, 1914. The sample and holdcircuit 1938 has a second signal input line 1956 from a second signalsource which is switchably coupled through respective shr2 reset sampleswitch 1910 and shs2 signal sample switch 1912 to first sides ofrespective capacitors 1920, 1918. The back side of respective capacitors1920, 1914 is switchably coupled to a first signal input line ofamplifier 1934 through column select switch 1926. The back side ofrespective capacitors 1918, 1916 is switchably coupled to a secondsignal input line of amplifier 1934 through column select switch 1928.Signal input line 1954 is coupled to a desired pixel, and signal inputline 1956 is coupled to a reference circuit.

A crowbar switch 1922 couples on one side the connection between shr1reset sample switch 1906 and capacitor 1916 and on the other side theconnection between shs1 signal sample switch 1908 and capacitor 1914. Asecond crowbar switch 1923 couples on one side the connection betweenshr2 reset sample switch 1910 and capacitor 1920 and on the other sidethe connection between shs2 signal sample switch 1912 and capacitor1918. A third crowbar switch 1921 couples on one side the connectionbetween capacitor 1920 and switch 1910 and on the other side theconnection between capacitor 1916 and switch 1906. A Vclamp voltage isswitchably coupled through switch 1930 to the conductive line betweenthe capacitor 1920 and the capacitor 1914. A Vclamp voltage is alsoswitchably coupled through switch 1932 to the conductive line betweencapacitor 1916 and capacitor 1918.

The first signal input line to the amplifier 1934, i.e., the conductiveline between the column select switch 1926 and amplifier 1934, iscoupled to the first output from amplifier 1934 through capacitor 1996.The first signal input line to the amplifier 1934 is also switchablycoupled to the first output from amplifier 1934 through switch 1986. Thesecond signal input line to the amplifier 1934, i.e., the conductiveline between the column select switch 1928 and amplifier 1934, iscoupled to the second output from amplifier 1934 through capacitor 1994.The second signal input line to the amplifier 1934 is switchably coupledto the second output from amplifier 1934 through switch 1988.

The operation of circuits of is now described with reference to thesimplified signal timing diagram of FIG. 16 (implementing a read from a3T pixel).

The Vclamp switches 1908, 1906 are enabled by a pulse clamp, therebyprecharging the capacitors 1914, 1916, 1918, and 1920. The shs1 signalsample switch 1908 and shs2 signal sample switch 1912 are pulsed bysignals shr1, 2, thereby temporarily enabling them. Thus, Vsig of adesired pixel and Vsig of a reference circuit are sampled and stored onrespective capacitors 1914, 1918. A pulse shr1, 2 is applied temporarilyenabling shr1 reset sample switch 1906 and shr2 reset sample switch1910. Thus, Vrst of a desired pixel and Vrst of a reference circuit aresampled and stored on respective capacitors 1916, 1920.

In order to read out the stored signals from the capacitors 1914, 1916,1918, and 1920, the amplifier 1934 is reset by temporarily closingswitches 1994 and 1996. A pulse ColSel signal is applied temporarilyclosing Col. Select switches 1926, 1928 thereby coupling capacitors1914, 1916, 1918, and 1920 to differential amplifier 1934. Crowbarswitches 1921, 1922, and 1923 are pulsed closed by signal CB therebyforcing the stored signals in capacitors 1914, 1916, 1918, and 1920through amplifier 1934. Amplifier 1934 outputs the resultingdifferential signals, thereby providing a fully differential output inwhich the common mode noise is eliminated from the desired outputsignal.

FIG. 20 illustrates another exemplary embodiment of a differentialcolumn storage and readout circuit 2000 of the present invention. Inthis embodiment an amplifier is placed in each column.

The sample and hold circuit 2038 has a first signal input line 2054 froma first signal source which is switchably coupled through s1 switch 2008to a first side of capacitor 2014. The back side of capacitor 2014 iscoupled to a first signal input line of amplifier 2034. The sample andhold circuit 2038 has a second signal input line 2056 from a secondsignal source which is switchably coupled through s2 switch 2006 to afirst side of second capacitor 2016. The back side of capacitor 2016 iscoupled to a second signal input line of amplifier 2034. Signal inputline 2054 is coupled to a desired pixel, and signal input line 2056 iscoupled to a reference circuit.

The first signal input line to the amplifier 2034, i.e., the conductiveline between the capacitor 2014 and amplifier 2034, is coupled to thefirst output from amplifier 2034 through capacitor 2096. The firstsignal input line to the amplifier 2034 is also switchably coupled tothe first output from amplifier 2034 through switch 2086. The secondsignal input line to the amplifier 2034, i.e., the conductive linebetween the capacitor 2016 and amplifier 2034, is coupled to the secondoutput from amplifier 2034 through capacitor 2094. The second signalinput line to the amplifier 2034 is also switchably coupled to thesecond output from amplifier 2034 through switch 2088.

The operation of circuit 2038 of is now described with reference to thesimplified signal timing diagram of FIG. 21 (implementing a read of a 3Tpixel).

The s1 switch 2008 and s2 switch 2006 are pulsed by signals s1, s2thereby enabling them. The switch 2086 and switch 2088 are pulsed bysignals fb1, fb2 thereby temporarily enabling them. Thus, Vsig of adesired pixel and Vsig of a reference circuit are sampled and stored onrespective capacitors 2014, 2016. The pulses fb1, fb2 are then disabled.Thus, Vrst of a desired pixel and Vrst of a reference circuit aresampled and stored on respective capacitors 2014, 2016. Then the pulsess1, 2 are disabled which forces the signals stored on the capacitors2014, 2016 through amplifier 2034.

If C is the capacitance of capacitors 2014, 2016, and Cf is thecapacitance of capacitors 2094, 2096 then the amplifier 2034 outputs:Vdiff=(C/Cf)(Vsig1−Vsig2+Vrst2−Vrst1).  (12)

Therefore the circuit 2038 provides a differential output.

FIG. 22 illustrates another exemplary embodiment of a differentialcolumn storage and readout circuit 2200 of the present invention. Thecircuit 2200 provides not only a differentiated signal output, but thesignal output is also symmetric.

Sample and hold circuit 2238 stores each pair of input signals on arespective capacitor. For example, the Vsig of the desired pixel inputon a first input line 2254 and the Vsig of the reference circuit inputon a second input line 2256 are stored on capacitor 2214. Additionally,the Vrst of the desired pixel and the Vrst of the reference circuit arestored on capacitor 2216. In this way the charge on the first capacitorisC1=½(C)(Vsig1−Vsig2−Vrst1+Vrst2).  (13)

and the charge on the second capacitor isC2=½(C)(Vsig1−Vsig2+Vrst1−Vrst2)  (14)

Consequently, the readout charge injected into the amplifier would bethe same voltage but oppositely charged. Additionally, the amplifieroutput provides elimination of a common noise component.

The sample and hold circuit 2238 has a first signal input line 2254 froma first signal source which is switchably coupled through respectiveshr1 reset sample switch 2206 and shs1 signal sample switch 2208 to afirst side of respective capacitors 2216, 2214. The sample and holdcircuit 2238 has a second signal input line 2256 from a second signalsource which is switchably coupled through respective shr2 reset sampleswitch 2210 and shs2 signal sample switch 2212 to a second side ofrespective capacitors 2216, 2214. The back sides of respectivecapacitors 2216, 2214 is respectively switchably coupled to amplifier2234 through clamp switches 2232, 2230. A crowbar switchably couplesthrough a switch 2222 between on one side the conductive line betweenshr1 reset sample switch 2206 and capacitor 2216 and on the other sideto the connection between the shs1 signal sample switch 2208 andcapacitor 2214. Signal input line 2254 is coupled to a desired pixel,and signal input line 2256 is coupled to a reference circuit.

The first signal input line to the amplifier 2234, i.e., the conductiveline between the switch 2230 and the amplifier 2234, is coupled to thefirst output from amplifier 2234 through capacitor 2296. The firstsignal input line to the amplifier 2234 is also switchably coupled tothe first output from amplifier 2234 through switch 2286. The secondsignal input line to the amplifier 2234, i.e., the conductive linebetween switch 2232 and amplifier 2234, is coupled to the second outputfrom amplifier 2234 through capacitor 2294. The second signal input lineto the amplifier 2234 is switchably coupled to the second output fromamplifier 2234 through switch 2288.

The operation of circuit 2238 is now described with reference to thesimplified signal timing diagram of FIG. 23 (using a 3T pixel as input)

The shs1 signal sample switch 2208 and shs2 signal sample switch 2212are pulsed by signals shs1, shs2 temporarily enabling them. Thus, Vsigof a desired pixel and Vsig of a reference circuit are sampled andstored on capacitor 2214. Pulses shr1, shr2 are applied temporarilyenabling shr1 reset sample switch 2206 and shr2 reset sample switch2210. Thus, Vrst of a desired pixel and Vrst of a reference circuit aresampled and stored on capacitor 2216.

The amplifier 2234 is then reset by applying pulses f1, f2 to switches2286, 2288 to temporarily enable them. To read out the signals stored oncapacitors 2216, 2214, the right plates of capacitors 2214, 2216 arecoupled to the amplifier 2234 while still in the reset state, byapplying a pulse clamp and temporarily enabling switches 2230, 2232.Then the signal stored on the capacitors 2216, 2214 are transferred whenthe switches 2286, 2288 are opened and applying a pulse CB closingcrowbar switch 2222. Thus the signals stored on the respectivecapacitors 2216, 2214 are forced through the amplifier 2234. Amplifier2234 outputs the resulting symmetric differential signals, therebyproviding a fully differential, symmetric output in which common modenoise is eliminated.

The method and apparatus aspects of the invention are embodied in animage device 2440 shown in FIG. 24 which provides an image outputsignal. The image output signal can also be applied to a processorsystem 2400, also illustrated in FIG. 24. A processor based system, suchas a computer system, for example, generally comprises a centralprocessing unit (CPU) 2410, for example, a microprocessor, thatcommunicates with one or more input/output (I/O) 2450 over a bus 2470.The CPU 2410 also exchanges data with random access memory (RAM) 2460over bus 2470, typically through a memory controller. The processorsystem may also include peripheral devices such as a floppy disk drive2420 and a compact disk (CD) ROM drive 2430 which also communicate withCPU 2410 over the bus 2470. Imager device 2440 is coupled to theprocessor system and includes a pixel storage and readout circuit asdescribed along with respect to FIGS. 5, 7, 9, 11, 13, 15, 17, 19, 20,and 22.

While the invention has been described and illustrated with reference tospecific exemplary embodiments, it should be understood that manymodifications and substitutions can be made without departing from thespirit and scope of the invention. Although the embodiments discussedabove describe specific numbers of transistors, photodiodes, conductivelines, the present invention is not so limited. Furthermore, many of theabove embodiments described are shown with respect to the operation ofthe sample and hold of a desired pixel that is a 3T pixel, the spirit ofthe invention is not limited to 3T pixels. Additionally, although theamplifier appears to be within, or associated with, a column readoutcircuit, the amplifier may be accessible globally. Accordingly, theinvention is not to be considered as limited by the foregoingdescription but is only limited by the scope of the claims.

1. A method of operating an imaging pixel array to provide adifferential output, said method comprising: storing a chargeaccumulated signal from a desired pixel and a comparison signal from areference circuit; storing a reset signal from the desired pixel and areset signal from the reference circuit; combining, in a column readoutcircuit, the charge accumulated signal from the desired pixel and thecomparison signal from the reference circuit; and combining, in thecolumn readout circuit, the reset signal from the desired pixel and thereset signal from the reference circuit.
 2. The method of claim 1,wherein said first storing act further comprises simultaneously storingthe charge accumulated signal from the desired pixel and the comparisonsignal from the reference circuit.
 3. The method of claim 2, whereinsaid second storing act further comprises simultaneously storing thereset signal from the desired pixel and the reset signal from thereference circuit.
 4. The method of claim 3, further comprising: storingin a first storage area, the reset signal from the desired pixel and thereset signal from the reference circuit; and storing, in a secondstorage area, the charge accumulated signal from the desired pixel andthe comparison signal from the reference circuit.
 5. The method of claim3, further comprising: storing, in a first storage area, the resetsignal from the desired pixel; storing, in a second storage area, thereset signal from the reference circuit; storing, in a third storagearea, the charge accumulated signal from the desired pixel; and storing,in a fourth storage area, the comparison signal from the referencecircuit.
 6. The method of claim 4, further comprising including a firstbias with the charge accumulated signal and the comparison signal. 7.The method of claim 6, further comprising including a second bias withthe charge accumulated signal and the comparison signal.
 8. The methodof claim 7, further comprising including a common mode feedback biaswith the charge accumulated signal and the comparison signal.
 9. Themethod of claim 4, wherein the desired pixel is a pixel having acapacitor.
 10. The method of claim 9, wherein the desired pixel is a 3Tpixel.
 11. The method of claim 9, wherein the desired pixel is a 4Tpixel.
 12. The method of claim 9, wherein the reference circuit is areference circuit having a capacitor.
 13. The method of claim 4, furthercomprising inputting the combined signals into an amplifier.
 14. Themethod of claim 13, wherein the amplifier is a differential amplifier.15. The method of claim 1, wherein the combined signals are symmetric.16. An imager system, comprising: a sample and hold circuit, comprising:a first circuit for sampling and holding a reset signal and an imagesignal from a desired pixel; a second circuit for sampling and holding areset signal and a comparison signal from a reference circuit; and areadout circuit coupled to said first and second circuits, said readoutcircuit being configured to combine said image signal from said desiredpixel and said comparison signal from said reference circuit, saidreadout circuit further configured to combine said reset signal fromsaid desired pixel and said reset signal from said reference circuit.17. The imager system of claim 16, wherein said first circuit furthercomprises first and second storage circuits for storing said respectiveimage and reset signals from said desired pixel.
 18. The imager systemof claim 17, wherein said second circuit further comprises third andfourth storage circuits for storing said respective comparison and resetsignals from said reference circuit.
 19. The imager system of claim 18,wherein said first and third storage circuits are switchably coupled toa first input line of an amplifier and said second and fourth storagecircuits are switchably coupled to a second input line of saidamplifier.
 20. The imager system of claim 18, wherein said first andfourth storage circuits are switchably coupled to a first input line ofan amplifier and said second and third storage circuits are switchablycoupled to a second input line of said amplifier.
 21. The imager systemof claim 18, wherein said first and second storage circuits are coupledto a first input line of an amplifier and said third and fourth storagecircuits are coupled to a second input line of said amplifier.